The need to improve hermetic package sealing of integrated circuits has led to the discovery of a new, low temperature brazing alloy. Presently, some device manufacturers are using an 80/20 gold-tin alloy to seal the lid of an integrated circuit to its package, but several have encountered difficulties in fabricating a hermetic seal acceptable for high reliability applications. Selection of a new hermetic lid sealant, therefore has become a problem of paramount importance. However, to improve the lid seal, a better understanding of the characteristics of the commercial solder alloys presently available is required.
Integrated circuit packages are conventionally sealed with low melting point materials including hard solders, soft solders and epoxies. Hard solders are alloys which are free from temperature fatigue failures while soft solders are subject to such failures. This distinction has restricted hard solders to the low melting gold eutectics while soft solders include practically all lead and tin-based alloys. Epoxies are used when the physical and chemical characteristics of the chip are adversely affected by the high temperature generated by the sealing operation. In general, when soft solders and epoxies are compared to hard solders, the soft solders and epoxies exhibit greatly inferior strength, thermal and adhesive properties. Accordingly, it has been generally concluded that hard solders give the best performance in sealing operations. Furthermore, the best hard solder presently available for sealing packages appears to be a 80/20 gold-tin eutectic alloy. However, it has been found that even this alloy does not always give satisfactory results in that it sometimes fails to seal the lid to the substrate.
The gold-tin eutectic alloy referred to above contains 80.0 weight percent gold, balance tin, and has a minimum melting temperature of 280.degree. C. Because of thermodynamic requirements, the gold-tin alloy would have to contain a greater percentage of gold than 80.0 weight percent in order to insure wetting of the gold-plated seal ring or substrate. However, at higher than 80.0 weight percent gold, the melting point of gold-tin alloys increases rapidly. The high melting point of such an alloy can result in poor wetting characteristics of the alloy and possible damage to the integrated circuit during the sealing operation.